27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.13 Class year_­month [time.cal.ym]

27.8.13.1 Overview [time.cal.ym.overview]

namespace std::chrono {
  class year_month {
    chrono::year  y_;           // exposition only
    chrono::month m_;           // exposition only

  public:
    year_month() = default;
    constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;

    constexpr chrono::year  year()  const noexcept;
    constexpr chrono::month month() const noexcept;

    constexpr year_month& operator+=(const months& dm) noexcept;
    constexpr year_month& operator-=(const months& dm) noexcept;
    constexpr year_month& operator+=(const years& dy)  noexcept;
    constexpr year_month& operator-=(const years& dy)  noexcept;

    constexpr bool ok() const noexcept;
  };
}
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year_­month represents a specific month of a specific year, but with an unspecified day.
year_­month is a field-based time point with a resolution of months.
year_­month meets the Cpp17EqualityComparable (Table 25) and Cpp17LessThanComparable (Table 26) requirements.
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year_­month is a trivially copyable and standard-layout class type.

27.8.13.2 Member functions [time.cal.ym.members]

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constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;
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Effects: Initializes y_­ with y, and m_­ with m.
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constexpr chrono::year year() const noexcept;
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Returns: y_­.
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constexpr chrono::month month() const noexcept;
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Returns: m_­.
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constexpr year_month& operator+=(const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Effects: *this = *this + dm.
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Returns: *this.
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constexpr year_month& operator-=(const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Effects: *this = *this - dm.
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Returns: *this.
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constexpr year_month& operator+=(const years& dy) noexcept;
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Effects: *this = *this + dy.
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Returns: *this.
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constexpr year_month& operator-=(const years& dy) noexcept;
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Effects: *this = *this - dy.
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Returns: *this.
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constexpr bool ok() const noexcept;
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Returns: y_­.ok() && m_­.ok().

27.8.13.3 Non-member functions [time.cal.ym.nonmembers]

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constexpr bool operator==(const year_month& x, const year_month& y) noexcept;
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Returns: x.year() == y.year() && x.month() == y.month().
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constexpr strong_ordering operator<=>(const year_month& x, const year_month& y) noexcept;
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Effects: Equivalent to: 
if (auto c = x.year() <=> y.year(); c != 0) return c;
return x.month() <=> y.month();
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constexpr year_month operator+(const year_month& ym, const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: A year_­month value z such that z.ok() && z - ym == dm is true.
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Complexity: with respect to the value of dm.
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constexpr year_month operator+(const months& dm, const year_month& ym) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: ym + dm.
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constexpr year_month operator-(const year_month& ym, const months& dm) noexcept;
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Constraints: If the argument supplied by the caller for the months parameter is convertible to years, its implicit conversion sequence to years is worse than its implicit conversion sequence to months ([over.ics.rank]).
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Returns: ym + -dm.
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constexpr months operator-(const year_month& x, const year_month& y) noexcept;
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Returns: 
x.year() - y.year() + months{static_cast<int>(unsigned{x.month()}) -
                             static_cast<int>(unsigned{y.month()})}
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constexpr year_month operator+(const year_month& ym, const years& dy) noexcept;
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Returns: (ym.year() + dy) / ym.month().
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constexpr year_month operator+(const years& dy, const year_month& ym) noexcept;
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Returns: ym + dy.
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constexpr year_month operator-(const year_month& ym, const years& dy) noexcept;
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Returns: ym + -dy.
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template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const year_month& ym);
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Effects: Equivalent to: 
return os << format(os.getloc(), STATICALLY-WIDEN<charT>("{}/{}"),
                    ym.year(), ym.month());
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template<class charT, class traits, class Alloc = allocator<charT>> basic_istream<charT, traits>& from_stream(basic_istream<charT, traits>& is, const charT* fmt, year_month& ym, basic_string<charT, traits, Alloc>* abbrev = nullptr, minutes* offset = nullptr);
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Effects: Attempts to parse the input stream is into the year_­month ym using the format flags given in the NTCTS fmt as specified in [time.parse].
If the parse fails to decode a valid year_­month, is.setstate(ios_­base​::​failbit) is called and ym is not modified.
If %Z is used and successfully parsed, that value will be assigned to *abbrev if abbrev is non-null.
If %z (or a modified variant) is used and successfully parsed, that value will be assigned to *offset if offset is non-null.
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Returns: is.